1. Field of the Invention
This invention relates generally to friction stir welding wherein heat for creating a weld is generated by plunging a rotating pin of a tool into a workpiece. More specifically, the present invention relates to performing crack repair, crack prevention, and material conditioning in a friction stir welding process that enables the present invention to repair or perform pre-emptive maintenance on materials that are welded, wherein the present invention is particularly applicable to the repair and maintenance of pipes, and has important applicability to the nuclear energy industry.
2. Description of Related Art
Crack formation and propagation is a serious problem in many applications that use metal and alloy structures and components. For example, pipes used in the exploration for and transportation of gas and petrochemicals are in constant need of replacement and repair. Furthermore, thousands of miles of new pipes are being manufactured and installed every year.
It is well known in the industry that crack initiation can be a result of loading, hydrogen embrittlement, corrosion, stress corrosion, material defects, and especially fusion welding defects. In particular, the process of fusion welding introduces residual stresses caused by solidification of a liquid phase, solidification cracking, porosity, large grain sizes prone to grain boundary corrosion, and crack initiation sites that originate from the heat affected zone. As a general rule of thumb, it can be assumed that fusion welding reduces the physical properties of the base metal by some 30% to 40%. The reason for the reduction in physical properties and creation of crack initiation sites is due to the introduction of defects and the altered grain structure of the metal or alloy from the fusion welding process.
While a need has been established to friction stir weld high melting temperature alloys for improved properties (making it possible to achieve 95%+ base metal properties), a methodology has not yet been established to repair cracks, except in aluminum alloys.
Accordingly, what is needed is a system and method for repairing cracks in metal matrix composites, ferrous alloys, non-ferrous alloys, and superalloys. What is also needed is a system and method for performing crack prevention on these materials and aluminum.
Another aspect of the invention relates to its applicability to use in environments that are difficult to perform any type of welding. For example, fusion welding is not a viable process for repairing welds in fluid or underwater environments. Fusion welding is even difficult to perform in air environments because of the cast structure that is created from a molten phase, and the accompanying defects. Fusion welding repair is at best a temporary solution to cracks since the process adds more defects to an already defect prone welding area.
Failure of pipes or other structures that are subject to the effects of crack propagation is a problem with serious consequences not only for the environment, but the health of workers. For example, consider a nuclear containment vessel. Leaks in such an environment can have serious health consequences for those involved.
Accordingly, it would be an advantage over the prior art to provide a system for friction stir welding in such environments, not only to repair cracks, but to also perform preventative maintenance.